X-ray tube



Oct. 16, 1934. A. BOUWERS X-RAY TUBE Fi1edllarch13,1930

9 766677 Baa/were), W

Patented (Jet. 16, 1934 PATENT OFFICE X-RAY TUBE Albert Bouwers, Eindhoven, Netherlands, as-

srgnor to N. V. Philips Gloeilampenfabrieken, Elndhoven, Netherlands Application March 13,

1930, Serial No, 435,604

In the Netherlands April 19, 1929 v 6 Claims.

The invention relates to X-ray tubes in which the anticathode and the electron beam move relatively with respect to one another. Such tubes present the great advantage that, with a given energy supplied to the tube the surface on which the electron beam strikes is loaded much less heavily than when the electron beam continuously strikes the same portion of the anticathode.

Owing to this, such tubes may be loaded much 5 more heavily or for a much longer time or again they may be constructed so as to have smaller dimensions for a given load.

In his prior specification No. 379,300 filed July 18, 1929 applicant has proposedalready to give the electronbeam in an X-ray tube with rotary anticathode, an elongated cross-sectional shape such that its longitudinal axis is perpendicular to the direction. of the movement, a tube being thus produced in which the advantages of a tube with movable anticathode are'combined with those of an electron beam of elongated crosssection. In this case the X-rays are caused to emerge from the tube at a very small angle with the surface so that sharp images can be ebtained.

In each revolution of the anticathode every point of the part of the surface which participates in the forming of the focal spot (that is, the spot onwhich the electron beam impinges), is loaded during a certain time, owing to which the material is heated. I

I have found that with the forms of electron beams hitherto used the heating is not uniform, that is: the heating is not the same at every point of the surface which participates in the forming of the focal spot. In itself this is not disadvantageous, but since the maximum load of the tube is determined by the highest temperature occurring at any point, it is only possible tov work with an energy smaller than that which corresponds to the uniform heating to the highest admissible temperature.

According to the invention, such a uniform heating is obtained by giving the focal spot a particular shape. The time interval during which; in each revolution or period of the relative movement of the anticathode with respect to the electron beam, a point'of the anticathode surface is loaded, depends on the speed at which .50. a point on the anticathode passes through the electron beam and on the length of the path described in the electron beam by the said point. When the various portions of the anticathode move at different speeds with relation to the electron beam, and when the paths described in the latter are of equal length, the material in the points moving with the greatest speed will be loaded during the shortest periods of time and will consequently be heated least. If, With a rotary anticathode the focal spot has a rectangular shape, this is the case with the part farthest remote from the axis of rotation. I

An X-ray tube according to the invention has an electron beam which conveniently has an elongated shape and whose longitudinal axis is preferably perpendicular to the direction of the mOvement. According to' the invention, the focal spot has everywhere or for a considerable portion such a dimension in the direction of the movement of the anode that it is traversed in substantially 7 equal periods of time. The tube may comprise a rotary anticathode and may have a focal spot whose breadth increases proportionately to the distance from the axis of rotation. In this case the focal spot is preferably limited by two lines which intersect each other in or near the axis of rotation.

The invention will be more clearly understood by referring to the accompanying drawing in which Figure 1 is a side View, partly in section, of an X-ray tube comprising a rotary anticathode according to the invention.

Fig. 2 is a perspective view of part of the anticathode and showing the shape of a focal spot as used heretofore.

Fig. 3 is a perspective View of part of the anticathode and showing the shape of a focal spot in accordance with the invention.

Figure 1 shows an X-ray tube whose outer wall consists of two'glass parts 1-1 which are surrounded by cylinders 2-2 of a material which is poorly transparent to X-rays. Between glass parts 1-1 is sealed a part 3 of metal preferably of form-chromium, which has a window 4 through which X-rays can emerge from the tube.

The tube comprises an incandescent cathode 5 arranged in a metal vessel 6. The ends of this incandescent cathode are connected to contact pins 7, one bymeans of asupply wire (not shown) 100 and the cylinder is ,3; tion and which the other via the metal vessel 6. To these contact pins may be connected the supply wires for the heating current. The beam of electrons emitted by the cathode, is concentrated by vessel 6, forming the focussing device, on a small area of the anticathode 8. The shape of the cross-sect 1 of the of cathode rays depends on the of the aperture 9 of the focussing device. Due to the ccentric position of the incandescent cathode and of the aperture 9, the cathode rays impact on a portion of the anticathode which is located on one side of the apex of the conical end-surface. The anticathode is rotatably secured to a spindle 10 in which terminates the metal member 11 which is sealed to the glass wall of the tube and which is connected to the contact member 20. The rotation of the anticathode is obtained due to the fact that it constitutes the rotor of an induction motor. For this purpose, it is constituted by a cylinder of good conducting material, for example copper, wlL'ch encloses another cylinder of a material having a high permeability.

The stator 15 of the motor is located entirely outside the tube. Magnet windings 17 on this stator furnish the field ofthe motor and may be connected to a multiphase alternating current network or, in conjunction with devices adapted to bring about a mutual phase-displacement, to an ordinary alternating current supply consequently connected to ground so that there is no possibility of coming into contact with the parts under tension. This cylinder surrounds the metal portion 3 of the Wall. A layer 19 of lead provided between this metallic portion prevents undesirable X-rays from emerging.

Figure 2 represents the slightly conical endsurface of the anticathode 8 of Fig. 1, the focal spot produced thereon during operation being indicated by 21. This is the place where the electrons emitted by the incandescent cathode impinge on the end-surface of the body 8 and from which the X-rays emanate. Its shape is determined by that of the aperture 9 of the focusslng device. The focal spot shown in Figure 2 has the shape of a rectangle whose longitudinal axis is located along a generatrix of the conical surface. If the width of this focal spot is called d, the time t during which, in each revolution, 2.

' point of the anticathode surface is located in the focal spot, is proportional to d, if r is the distance from the point under consideration to the apex of the cone. It is evident that the increase of temperature brought about by the load will i i be the greater, the higher is the value of t. If

now the highest admissible temperature is T1, the maximum admissible load will be the load which causes the temperature of that portion of the focal spot which is nearest the axis of rotain every revolution is loaded during a time n, to rise to the said value T1 because the temperatures, produced in other portions of the focal spot with a larger 1* and consequently a smaller t, are lower.

A greater energy would be arrived at by loading the anticathode in such manner that at the other points of the focal spot the temperature also rises to the value T1. This result can be obtained by making the duration of the load for these points in each revolution equal to h;

it is possible to influence t by varying d. If (1 is made proportional to 1',

is a constant number, and t is consequently constant. In this case the heating is also uniform and the highest efiiciency is obtained.

As in the mode of realization above described the focal spot is formed on the conical end-surface of the anticathode, this theoretically ideal focal spot is consequently limited by two straight lines which intersect each other in the axis of rotation, namely by two generatrices of the conical surface. Modes of realization, however, in which such is not exactly the case should notwithstanding be considered to fall within the scope of the invention. Nor it is absolutely necessary that the short sides of the focal spot are portions of circles around the axis, the focus may also be rounded off on one or on both sides. It is essential that the proportionality of the breadth and the distance from the axis exists over a considerable portion of the lengths of the focal spot. The illustrated shape of the focal spot is also favourable in view of the emerging beam of rays.

The portion of the conical surface which in each revolution turns through the beam of cathode rays may consist of a tungsten ring having at any point the same thickness.

With tubes as above described it is generally very advantageous that theanticathode mirror be very thin, for example less than 0,3 mm., for in this case the load may be such that both the surface of the tungsten and the surface of separation with the material lying under it, which preferably is copper, are thermally loaded at a maximum; the efi'iciency of the tube in this case being as high as possible.

What I claim is:

1. An X-ray tube having an envelope, a rotatable anode, a .cathode, and a focussing device for said cathode, and having an aperture therein, the width of the said aperture at any point along its side being directly proportional to the distance of said point from the axis of rotation of the anode.

2. In an X-ray tube, an envelope, a rotatable anode having a cylindrical portion and a conical front surface, a cathode adapted to be heated to incandescence and a device cooperating with said cathode for focussing the beam of cathode rays, said cathode facing a conical front surface of the anode and being slightly displaced laterally from the axis of rotation of the anode, the focussing device having an elongated aperture for the passage of the electrons, which is'wider at the side remote from said axis than at the side adjacent said axis.

3. The method of uniformly loading a rotatable anode by an electron beam, comprising projecting a beam of electrons thereon having a width an electron beam, comprising projecting a beam of electrons on the anode, the projected area of the beam on the anode having for at least a portion of its extent, a trapezoidal shape, the major axis of which trapezoid coincides with the major axis of the projected area and forms a plane with the major axis of the anode.

6. A method of loading a rotatable anode by an electron beam, comprising projecting a beam, of electrons thereon, the width of said beam in any point ofa section perpendicular to the cathode rays, increasing linearly at least over a considerable portion of the length of said section, with increase in the distance outwardly from the axis of the rotation of the anode.

ALBERT BOUWERS, 

